PHASE-TRANSITION AND COMPLEX MOTION OF IONS IN PYRIDINIUM HEXAFLUOROPHOSPHATE

Proton and fluorine NMR second moments and spin-lattice relaxation times are studied for polycrystalline pyridinium hexafluorophosphate over a wide range of temperature. A solid-solid phase transition discovered at 217 K is confirmed by DTA. An analysis of all cross-relaxation effects enables us to derive a model of cation and anion reorientations in both phases and to obtain respective activation parameters. In the low-temperature phase a complex motion of the cation over inequivalent barriers is described by two rotational correlation times. One of them appears to be effective for driving the phase transition due to its convergence with the anion correlation time. Gradual fading of the inequivalence in potential barriers leads to a simple reorientation of the cation above the phase transition.